Multilayer printed circuit boards have traditionally comprised a stack of individual printed circuit boards or innerlayers separated by dielectrical material. The circuitry of the several innerlayers is electrically connected by bored and plated through-holes. Multilayer printed circuit boards provide circuitry in a three-dimensional array and are therefor advantageously space-saving, relative to individual printed circuit boards, which provide at most two layers of circuitry on a two-sided board.
These printed circuit boards are commonly provided with internal ground and power planes. These internal planes are frequently solid sheets of copper interrupted only by clearance holes (the perforations required for electrically isolating the through-hole pattern of the printed circuit board). Ground and power planes provide power voltage and current and ground connections for the components of the multilayer printed circuit. A second function of the ground and power planes is to provide electromagnetic shielding for the multilayer printed circuit board and reduce the electromagnetic and radio frequency interference. Multiple ground and power planes and additional ground planes on the surface layers with the conductive pattern are common.
Multilayer circuits enable formation of multiple circuits in minimal volume. They typically comprise a stack of layers with layers of signal lines (conductors) separated from each other by dielectric layers having plated through-holes known as vias providing electrical interconnections between the layers.
Current processes for fabricating multilayer boards are extensions of methods used for fabricating double-sided boards. The method comprises fabricating of separate innerlayers having circuit patterns disposed over their surface. A photosensitive material is coated over the copper surfaces of a copper clad innerlayer material, imaged, developed, and etched to form a conductor pattern in the copper cladding protected by the photosensitive coating. After etching, the photosensitive coating is stripped from the copper, leaving the circuit pattern on the surface of the base material. Following formation of the innerlayers, a multilayer stack is formed by preparing a lay up of innerlayers, ground plane layers, power plane layers, etc., typically separated from each other by a dielectric prepeg (a layer consisting of glass cloth impregnated with partially cured material, typically a B-stage epoxy resin). The outer layers of the stack comprise copper-clad, glass-filled epoxy board material with the copper cladding comprising exterior surfaces of the stack. The stack is laminated to form a monolithic structure using heat and pressure to fully cure the B-stage resin.
Interconnections or through-holes, buried vias, and blind hole interconnections are used to connect circuit layers within a multilayer board. The buried vias are plated through-holes connecting two sides of an innerlayer. Blind vias typically pass through one surface of the stack and pass into and stop within the stack. Regardless of the form of interconnections, holes are generally drilled at appropriate locations through the stack, catalyzed by contact with a plating catalyst and metallized, typically with electroless copper that is overplated with electrolytic copper, to provide electrical contact between circuit innerlayers.
The uses, advantages and fabricating techniques for the manufacture of multilayer boards are described by Coombs, Printed Circuits Handbook, McGraw Hill Book Company, New York, 2nd edition, pp. 20-3 to 23-19, 1979, incorporated herein by reference.
Multilayer boards have become increasingly complex. For example, boards for main frame computers may have as many as 36 layers of circuitry, with the complete stack having a thickness of about 1/4 inch. These boards are typically designed with 4 mil wide signal lines and 12 mil diameter vias for interconnections between signal line layers. For increased densification, it is desired to reduce signal lines to a width of 2 mils or less and vias to a diameter of 2 to 5 mils or less.
The photoimageable dielectric coatings for printed circuit boards on the leading edge of the technology must be capable of being processed in a minimum number of steps. Their dielectric and photolithographic properties, flexibility, and intercoat adhesion also must be excellent. A low photospeed, high moisture resistance, and good adhesion to a plated metal are also important properties of such coatings.
This invention is directed to negative-acting photosensitive dielectric compositions. The processing of negative-acting photoresists generally follows the sequence of applying a solution of the resist to a copper foil laminated to an epoxy resin base, drying and baking the resist to expel the solvent, exposing the resist to actinic radiation through a patterned photomask to define an image, dissolving the non-exposed portions of the resist in a developer, such as an alkaline aqueous developer to delineate the irnage, rinsing, and in some instances post-baking the imaged resist. The areas void of resist are then either etched, plated with metal, or filled with a conducting polymer. As discussed above, the use of copper foil is eliminated by the method of this invention.
This invention's currently preferred embodiments are closely akin to the invention described in U.S. Ser. No. 08/801,682 filed 18 Feb. 1997. The invention described therein is directed to a positive-acting photoimageable composition which has a similar function to the photoimageable composition of the present invention. The positive-imaging composition in that application contains a hydroxy-functional novalac resin, a cross-linkable resin, such as an epoxy resin, a naphthoquinone diazide, and at least one thermally labile halogen-containing cure catalyst. The novalac resin is normally soluble in alkaline aqueous solution, such as a sodium hydroxide solution. However, the naphthoquinone diazide acts to inhibit the novalac resin from being dissolved until the naphthoquinone diazide is exposed to actinic radiation; whereupon, the naphthoquinone diazide undergoes a rearrangement such that it facilitates the dissolving of the novalac resin in alkaline aqueous solution. The cross-linkable resin, particularly as catalyzed by the cure catalyst, undergoes a post development cure that renders the layer hard and permanent.
While the above-identified application details advantage of positive-acting photoresists, there are also advantages to negative-acting resists. Negative-acting resists tend to have faster photospeeds. Also, in formulations of the type discussed in the above-identified application and in the instant application, there tends to be out-gassing in positive-acting resists, whereas negative-acting resists of the type described herein do not exhibit outgassing.
It is an object of this invention to provide a novel method for manufacturing multilayer printed circuit boards by the selective plating of the dielectric layers, eliminating the need for the standard copper foil inner layers.
It is another related object of this invention to provide a negative-acting, photoimageable dielectric composition whose post-develop image is highly stable, both chemically and thermally.
It is still another object of this invention to provide a multilayer printed circuit board having permanent innerlayers made of a negative-acting photoimaged dielectric composition.
It is yet another object of this invention to provide a method for making a multilayered printed circuit board by which the vias are photodefined.
These and other objects of the invention which will become apparent from the following description of the invention.